A typical hydraulic shock dampening device (hereafter damper) comprises essentially: a working cylinder filled with hydraulic fluid (hereafter oil); and a gas-charged reservoir adjacent the working cylinder which receives and dispenses oil from and to the working cylinder during the damper's compression and rebound strokes respectively. The working cylinder comprises a damping piston axially slidable within a close fitting surrounding tube. The piston has appropriate passages therethrough and damping valves therein which control the flow of oil from one side of the piston to the other as it reciprocates within the tube. One end of the working cylinder is sealed shut and about the piston rod while the other end communicates with the reservoir to permit oil to shuttle back and forth between the working cylinder and the reservoir as required. The reservoir contains a chamber full of a cushioning gas which compresses during the damper's compression stroke and expands during the damper's rebound stroke in order to accommodate the otherwise incompressible oil flowing from and to the working cylinder. The expansion and compression of the gas cushion also accommodates the expansion and contraction of the hydraulic oil at various temperatures. In so-called "single tube" dampers, the reservoir is aligned axially with the working cylinder and is usually defined by a simple extension of the same tube that defines the working cylinder. In so-called "double tube" dampers, the working cylinder is surrounded by a radially spaced second tube (known as the reservoir tube) and the annular space therebetween becomes the reservoir.
It is known to physically separate the oil from the cushioning gas in the reservoir in order to prevent the gas from aerating and foaming the oil. In this regard, aeration of the oil with cushioning gas is the primary cause of a condition known as "lag" which is lost motion occurring in the damping piston due to the presence of compressible hydraulic fluid (i.e., the aerated oil) in the working cylinder. One manufacturer (i.e., of single tube dampers), positions a floating piston between the gas and the oil in the reservoir to isolate one from the other. Other manufacturers package the cushioning gas in a hermetically sealed, gas-filled bladder which is submerged in the oil in the reservoir. One such bladder-containing damper of the double-tube type is disclosed in Stultz U.S. Pat. No. 3,024,875, which is assigned to the assignee of the present invention and is specifically incorporated herein by reference. Generally speaking Stultz discloses a Freon-filled bladder made from thin nylon or Mylar sheets sealed together along their edges. Assemblywise, Stultz's bladder is: filled with approximately one atmosphere (i.e., room temperature and rod extended) of Freon; curled; and inserted into the reservoir tube. Thereafter, the working cylinder is inserted into the center of the curled bladder and oil poured to overflowing into both the cylinder and the reservoir. The cylinder and reservoir tubes are then capped and welded closed. The cap for the working cylinder includes appropriate passages and valving for communication with the surrounding reservoir. In more recent years SF.sub.6 gas has been used in place of Freon.
It is also known to pressurize the cushioning gas pocket to superatmospheric pressures ranging from about 2 to about 20 atmospheres. Pressurization helps to reduce lag due to reservoir gas-oil mixing (i.e., in bladderless dampers) and also reduces a condition known as "cavitational" lag which otherwise occurs in both bladder-type and bladderless dampers. Cavitational lag results from vaporization of the oil in the damper and subsequent aeration of the oil with the oil vapor. Superatmospheric pressurization not only reduces the amount of vapor that is formed but also so shrinks the size of the bubbles that are formed as to render them virtually harmless, lag-wise. Finally, superatmospheric pressurization imparts gas-spring characteristics to the damper which supplement the primary springs of the vehicle for improved control and handling of the vehicle.
A number of techniques for superatmospherically pressurizing dampers are known. One technique, for example, charges a single tube damper (i.e., with axially aligned reservoir and a floating oil-gas separating piston) by means of a special fixture sealingly secured to the open end of the tube defining the cylinder and reservoir. The fixture holds the floating and working pistons away from the mouth of the tube while the cushioning gas is pumped into the tube to a predetermined first pressure. Thereafter: (1) the floating and working pistons are pushed into the tube to compress the gas to a second predetermined working pressure; (2) the operating oil is pumped in behind the working piston; (3) the tube is sealed closed; and (4) the special fixture is removed. Another technique fully assembles a bladderless double-tube damper and then pressurizes it by: piercing the reservoir tube; injecting the cushioning gas under pressure into the reservoir; and finally welds shut the pierced hole. Still another technique (i.e., for bladder-type, double-tube dampers) provides a special sealable fitting through the wall of the reservoir tube and the bladder through which cushioning gas is pumped into the bladder from an external source after the damper has been filled and sealed (e.g., see Duckett U.S. Pat. No. 3,945,663).
The aforesaid pressurizing techniques are not only unnecessarily complicated and require special equipment but cannot be used effectively to pressurize bladder-type dampers such as Stultz U.S. Pat. No. 3,024,875 (supra) where the bladder is completely submerged in the reservoir oil so as to be incommunicado the outside of the damper. Moreover, existing practices for stuffing low pressure (i.e., atmospheric) bladders into reservoirs prior to filling and sealing cannot be used with bladders which are distended by high prepressurization. This is especially true with Stultz-like bladder-type, double-tube dampers where the bladder is curled to facilitate stuffing and positioning around the working cylinder. Even if it were practical to properly position highly prepressurized bladders in the reservoirs, special equipment would then be necessary to fill the dampers with pressurized oil and to seal them while still under pressure.
Accordingly, it is an object of the present invention to provide an ultrasimple method for manufacturing a high pressure, shock dampening device wherein superatmospheric pressure cushioning gas is generated in situ after the device is assembled by the thermal decomposition of a substance placed in the device during assembly. It is a further object of the present invention to provide a superatmospheric pressure, bladder-type shock dampening device wherein the bladder is completely submerged in the oil and incommunicado the exterior of the device. This and other objects and advantages of the present invention will become more readily apparent from the detailed description thereof which follows.